Cardiac muscle contraction is powered by actomyosin ATPase that is regulated by Ca2+ binding to the troponin complex (references 1, 2, see below). A cardiac muscle cell contains multiple myofibrils that are composed of repeating contractile units called sarcomeres. A sarcomere is made up of partially overlapping assemblies of myosin filaments (the thick filaments) and actin filaments (the thin filaments) (Cooke, 1986; Leavis & Gergely, 1984; reviews). Actin-activated myosin ATPase (i.e., the actomyosin ATPase powers muscle contraction in a process regulated by Ca2+ binding to the thin filament-associated proteins, tropomyosin and the troponin complex (Gordon et al., 2000; review). The current model for striated muscle (i.e., cardiac and skeletal muscle) contraction has contraction initiated by a rise in the cytoplasmic [Ca2+], which results in binding of Ca2+ to troponin C (TnC). Ca2+-TnC binding induces a series of allosteric changes in TnC, TnI, TnT and tropomyosin. These conformational changes allow the myosin head to form a strong cross-bridge with the actin filament. This interaction activates the myosin ATPase, displacing the thin filaments relative to the thick filaments and thus leading to a shortening of the sarcomere and contraction of the muscle (Cooke, 1986). TnI is the inhibitory subunit of troponin and plays a critical role in this Ca2+-signaling system (Perry, 1999; review).
The troponin complex consists of three subunits: troponin C (TnC, the Ca2+-binding subunit), troponin T (TnT, the tropomyosin-binding subunit), and troponin I (TnI, the inhibitory subunit) (2, 3). In addition to the core structure conserved in all TnI isoforms, cardiac TnI (cTnI) has an approximately 30-amino-acid N-terminal extension that is not present in fast and slow skeletal muscle TnIs. This N-terminal extension does not contain binding sites for other thin filament proteins (3, 4), but contains serine residues 23 and 24 which are protein kinase A (PKA) substrates. With β-adrenergic stimulation, phosphorylation of these serine residues facilitates myocardial relaxation by decreasing the affinity of TnC for Ca2+ (5, 6).
In a rat tail-suppression model simulating the effect of weightlessness on the cardiovascular system, an N-terminal truncated cTnI was found to be up-regulated in the heart after three to four weeks of simulated weightlessness (7). This truncated cTnI is produced by restricted proteolysis, which removes the N-terminal amino acids 1-30. The restricted cleavage of cTnI selectively deletes the cardiac-specific N-terminal extension, including the regulatory serine residues, but leaves the core structure intact (7).
Long-term exposure to a weightless environment results in decreased cardiac function (8-10). A number of cardiovascular adaptations to weightlessness in microgravity and simulated microgravity occur in response to redistribution of body fluids to the head and neck region (11-16). In long-term exposure to microgravity and simulated microgravity, this fluid redistribution results in increased renal discharge of Na+ and water to reduce circulatory blood volume and central venous pressure. According to the Frank-Starling relationship, cardiac function decreases as a result of this decreased preload. It is, therefore, important to determine whether proteolytic removal of the cTnI N-terminal domain has a negative impact on myocardial contractility or whether it is a compensatory response to the decrease of cardiac preload.
Characterization of the post-translational mechanisms in myocardial adaptation to long-term exposure to microgravity will provide a better understanding of myocardial dysfunction in astronauts, as well as in bedridden patients in which similar cardiovascular changes occur (9). The N-terminally truncated cTnI is present in normal hearts of all species examined (7). Understanding the functional effect of this structural modification of cTnI may increase our knowledge of mechanisms regulating myocardial contraction.
Thus, a need continues to exist in the art for methods of increasing the ventricular relaxation of the heart, resulting in an increased chamber filling and blood flow in accordance with the Frank Starling mechanism, thereby addressing such cardiac diseases, disorders and conditions as cardiac failure. Such methods should produce few, if any, side effects to maximize their value as therapeutics for the treatment of circulatory system disorders, such as cardiac diseases and disorders in humans.